Download Random Mutations and Evolutionary Change: Ronald Fisher, JBS

evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_19&context=...
Random Mutations and Evolutionary Change: Ronald Fisher, JBS Haldane, & Sewall Wright
For 70 years after the publication of the Origin of Species, it seemed as if Lamarck's shadow would loom forever over Darwin. On the one hand, most biologists came to the reality of evolution
— that living species shared a common ancestry and had been transformed over time. But natural selection — the engine of evolution, according to Darwin — remained controversial. Many
biologists argued that there must be some built-in "direction" to the variation that arose in each generation, helping to push each lineage towards its current state.
Many of these first geneticists who rediscovered Mendel's insights around 1900 also opposed natural selection. After all, Darwin had talked of natural selection gradually altering a species by
working on tiny variations. But the Mendelists found major differences between traits encoded by alleles. A pea was smooth or wrinkled, and nothing in between. In order to jump from one
allele to another, evolution must make giant jumps—an idea that seemed to clash with Darwin.
Natural selection in a Mendelian world
But in the 1920s geneticists began to recognize that natural selection could indeed act on genes. For one thing, it became clear that any given trait was
usually the product of many genes rather than a single one. A mutation to any one of the genes involved could create small changes to the trait rather than
some drastic transformation. Just as importantly, several scientists — foremost among them Ronald Fisher (above left), JBS Haldane (above right), and
Sewall Wright (below right) — showed how natural selection could operate in a Mendelian world. They carried out breeding experiments like previous
geneticists, but they also did something new: they built sophisticated mathematical models of evolution.
Small, not drastic, changes
Known as "population genetics," their approach revealed how mutations arise and, if they are favored by natural selection, can spread through a population. Even a slight
advantage can let an allele spread rapidly through a group of animals or plants and drive other forms extinct. Evolution, these population geneticists argued, is carried out
mainly by small mutations, since drastic mutations would almost always be harmful rather than helpful.
Wright introduced the most compelling metaphor in population genetics, known as the "adaptive landscape" (see figure, below). You can imagine the varying fitness of
different combinations of genes as a hilly landscape, in which the valleys represent less-fit combinations of genes and the peaks represent the fitter ones. Natural selection
tends to move the populations towards the peaks of the hills. But since the environment is always changing, the peaks shift, and the populations follow after them in a
never-ending evolutionary journey.
1 di 2
21/04/2010 11.30
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_19&context=...
Natural selection in the wild
Population genetics became one of the key elements of what would be called the Modern Synthesis. It showed that natural selection could produce
evolutionary change without the help of imaginary Lamarckian forces. Scientists have used the mathematical tools developed by Fisher, Wright, and
Haldane to measure evolutionary change in the wild with exquisite precision. Their insights have even allowed medical researchers to decipher the
puzzle of some hereditary diseases. Sickle-cell anemia, for example, is caused when children inherit two defective copies of a gene involved in making
hemoglobin. But a single copy of this allele can give some protection against malaria (see figures, right). Natural selection finds a balance between the
reproductive disadvantage of being born with two copies of the allele and the advantage of having one. Genetic disorders such as sickle-cell anemia are
actually the agonizing byproduct of natural selection acting on our ancestors.
Top:
Distribution of
malaria in
southern
Europe,
southwest Asia,
and Africa
around 1920
(green), prior to
mosquito
eradication
programs.
Bottom:
Distribution of
the sickle-cell
allele within the
same area. The
darker the
blue,the greater
the percentage
of people
carrying the
allele. Note the
correlation
between these
maps.
View this article online at:
http://evolution.berkeley.edu/evolibrary/article/history_19
Fisher image courtesy of the School of Mathematics and Statistics, University of St. Andrews, Scotland; Haldane image courtesy of the American Philosophical Society; Wright image © Hildegard Adler; "Adaptive landscape" image after a graphic by Rodney Dyer,
Iowa State University
Understanding Evolution © 2010 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California
2 di 2
21/04/2010 11.30
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_20&context=...
Starting "The Modern Synthesis": Theodosius Dobzhansky
Ronald Fisher and his colleagues set Darwin's concept of natural selection on a new foundation of genetics. They left an equally major project open for later biologists: to
explain in the language of genes, what species are and how they originate. The answer only began to emerge in the 1930s, thanks in large part to the work of a Soviet-born
geneticist named Theodosius Dobzhansky (right).
Dobzhansky, who emigrated to the United States in 1928, worked in Thomas Hunt Morgan's "Fly Room," where mutations were being studied closely for the first time. He
also paid careful attention to the work of population geneticists such as Sewall Wright, who were showing how the size of a population affects the rate at which a mutation
can spread. Dobzhansky was interested in discovering the genetics that determined the differences between populations of a species.
Genetically variable populations
At the time, most biologists assumed that all of the members of any given species had practically identical genes. But these were assumptions bred in the lab. Dobzhansky
began analyzing the genes of wild fruit flies, traveling from Canada to Mexico to catch members of the species Drosophila pseudoobscura. He found that different populations of D.
pseudoobscura did not have identical sets of genes. Each population of fruit flies he studied bore distinctive markers in its chromosomes that distinguished it from other populations.
Dobzhansky helped discover that
different fruit fly populations have
different frequencies of two
different versions of the same
chromosome; chromosome A
might be more frequent in one
population while chromosome A'
is more frequent in a neighboring
population.
If there was no standard set of genes that distinguished a species, what kept species distinct from each other? The answer, Dobzhansky correctly realized, was sex. A species is simply a group
of animals or plants that reproduces primarily among themselves. Two animals belonging to different species are unlikely to mate, and even if they do, they will rarely produce viable hybrids.
Dobzhansky ran experiments on fruit flies that demonstrated that this incompatibility is caused by specific genes carried by one species that clash with the genes from another species.
Making a new species
In 1937, Dobzhansky published these results in a landmark book, Genetics and the Origin of Species. In it, he sketched out an explanation for how
species actually came into existence. Mutations crop up naturally all the time. Some mutations are harmful in certain circumstances, but a
surprising number have no effect one way or the other. These neutral changes appear in different populations and linger, creating variability that is
far greater than anyone had previously imagined.
This variability serves as the raw material for making new species. If the members of a population of flies should breed among themselves more
than with other members of the species, their genetic profile would diverge. New mutations would arise in the isolated population, and natural
selection might help them to spread until all the flies carried them. But because these isolated flies were only breeding within their own population,
the mutations could not spread to the rest of the species. The isolated population of flies would become more and more genetically distinct. Some
of their new genes would turn out to be incompatible with the genes of flies from outside their own population.
If this isolation lasted long enough, Dobzhansky argued, the flies might lose the ability to interbreed completely. They might simply become unable
to mate with the other flies successfully, or their hybrid offspring might become sterile. If the flies were now to come out of their isolation, they
could live alongside the other insects but still continue mating only among themselves. A new species would be born.
The Modern Synthesis
1 di 2
21/04/2010 11.35
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_20&context=...
Dobzhansky's ability to combine genetics and natural history attracted many other biologists to join him in the effort to find a unified explanation
of how evolution happens. Their combined work, known as "The Modern Synthesis," brought together genetics, paleontology, systematics, and
many other sciences into one powerful explanation of evolution, showing how mutations and natural selection could produce large-scale
evolutionary change. The Modern Synthesis certainly did not bring the study of evolution to an end, but it became the foundation for future
research.
View this article online at:
http://evolution.berkeley.edu/evolibrary/article/history_20
Dobzhansky image courtesy of the American Philosophical Society Library
Understanding Evolution © 2010 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California
2 di 2
21/04/2010 11.35
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_21&context=...
Speciation: Ernst Mayr
Dobzhansky's Genetics and the Origin of Species captivated biologists far beyond the confines of genetics. In the mountains of New Guinea, an ornithologist named Ernst
Mayr (right) found the book to be an enormous inspiration. Mayr specialized in discovering new species of birds and mapping out their ranges. It is no easy matter
determining exactly which group of birds deserves the title of species. A bird of paradise species might be recognizable by the color of its feathers, but from place to place, it
might have a huge amount of variation in other traits — on one mountain it might have an extravagantly long tail while on another its tail would be cut square (below right).
Variation between populations
Biologists typically tried to bring order to this confusion by recognizing subspecies — local populations of a species that were distinct
enough to warrant a special label of their own. But Mayr saw that the subspecies label was far from a perfect solution. In some cases,
subspecies weren't actually distinct from each other, but graded into each other like colors in a rainbow. In other cases, what looked
like a subspecies might, on further inspection, turn out to be a separate species of its own.
Like many other naturalists of his day, Mayr suspected at first that some kind of Lamarckian heredity might be at work in evolution. But
when he read Dobzhansky and other architects of the Modern Synthesis, he realized that it was possible to explain the origin of species
with genetics. Mayr also realized that the puzzle of species and subspecies shouldn't be considered a headache: they were actually a
living testimony to the evolutionary process Dobzhansky wrote about. Variations emerge in different parts of a species' range, creating
differences between populations (see example below). In one part of a range the birds may possess long tails, in others, square tails.
But because the birds also mate with their neighbors, they do not become isolated into a species of their own.
The tails of birds of paradise living in the
mountains of western New Guinea (A)
are longer than those of birds living in
the more central mountains (B).
The size and shape of Dicrurus paradiseus' crest varies considerably
across southeast Asia.
1 di 2
21/04/2010 11.37
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_21&context=...
Geographic isolation
A population of birds, or any organism, can speciate if isolated from its neighbors. In his 1942 book, Systematics and the Origin of Species, Mayr argued that the most significant way to cut off
a population is by geographical isolation (see illustration at right). For example, a glacier may thrust down a valley, creating two separate populations, one on either side of the glacier. A rising
ocean may turn a peninsula into a chain of islands, stranding the beetles on each of them. This sort of isolation doesn't have to last forever; it needs only form a barrier long enough to let the
isolated population become genetically incompatible with the rest of its species. Once the glacier melts, or the ocean drops and turns the islands back into a peninsula, the animals will be
unable to interbreed. They will live side by side, but follow separate evolutionary fates.
Other modes of speciation
Today, scientists studying the origin of species can compare not just the bodies of species, but their genes as well. Geographic isolation remains a crucial element in forming new species, but
a number of biologists now argue that the formation of species can take several different paths. It may be possible, for example, for a population to continue breeding with other members of
its species — and trading genes — while still diverging into a distinct group. All that may be required is that a few of its genes diverge, thanks to strong natural selection. If the conditions are
right, this genetically distinct population may then become a new species.
Others argue that organisms can diverge into genetically distinct populations even if they are living side by side. For example, females may be born with different preferences for mates, and
those preferences may get strengthened over time into reproductive isolation. Even as biology's understanding of species formation evolves, Mayr's work remains hugely important to the
understanding of how the millions of species on Earth came to be.
View this article online at:
http://evolution.berkeley.edu/evolibrary/article/history_21
Mayr image courtesy of the Ernst Mayr Library, Harvard University; Bird of paradise tails after an illustration in Mayr, E. 1942. Systematics and the Origin of Species. Columbia University Press, New York; Dicrurus graphic after an illustration in Futuyama, D.J. 1986.
Evolutionary Biology. 2nd ed. Sinauer Associates, Inc.
Understanding Evolution © 2010 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California
2 di 2
21/04/2010 11.37
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_22&context=...
DNA, the Language of Evolution: Francis Crick & James Watson
DNA may be the most famous molecule in the world today, but it came to the attention of scientists rather late in the history of biology. Gregor Mendel found some of the underlying
regularities of heredity almost a century before DNA was discovered. At the turn of the century scientists discovered similar principles then rediscovered Mendel's work and rapidly realized that
life was somehow encoded in genes. Just what those genes were made of was a mystery, but that did not prevent scientists from starting to work out the dynamics of genes and mutations, and
how new forms of life could result from natural selection. The Modern Synthesis of evolution, the foundation on which most research on evolution has rested for the past 50 years, was already
set in place years before DNA was discovered.
The structure of DNA
But there's no denying that the discovery of DNA was a tremendous milestone in the exploration of evolution. While evolutionary biologists were fashioning the Modern
Synthesis, geneticists around the world searched furiously for the molecules that carried genetic information. They knew that cells contained several different types of
molecules, such as proteins and nucleic acids. But which had the capacity to bear information and be copied into new cells? Experiments showed that nucleic acids could
affect hereditary traits. A young American geneticist named James Watson (left) was one of the researchers who realized that the only way to determine whether they did in
fact carry genes was to understand their structure.
This was an agonizing task because scientists could only see molecules by shining x-ray beams on them, which then bounce off the atoms and strike a
piece of film in various distinctive patterns. At Cambridge University he joined up with Francis Crick (right) to analyze the x-ray data collected by Rosalind Franklin and
others. In a sudden burst of insight, Watson and Crick built a model out of brass plates and clamps and other bits of laboratory equipment in 1953. As they worked, they
realized that nucleic acids are arranged on a twisted ladder, with two runners made of phosphates and sugars, and a series of rungs made of pairs of organic compounds
known as bases. Years later, they won the Nobel Prize for this frenzy of discovery of DNA's double helix.
Life's cookbook
In the years that followed, Watson, Crick, and other researchers figured out the basics of how DNA works. Each gene, they
realized, consists of a stretch of base pairs. A single-stranded copy of the gene was created (known as messenger RNA) and
transported to protein-building factories in the cell called ribosomes. There, the sequence of the bases guided the assembly of
a string of amino acids that became a new protein. When a cell divides, the double helix is unzipped and the DNA is
replicated. It is life’s cookbook.
Using DNA
Evolutionary biology was revolutionized by the discovery of DNA. Mutations, researchers realized, change the spelling of the
cookbook. A single base pair may change, or a set of genes may be duplicated. Those mutations that confer a selective
advantage to an individual become more common over time, and ultimately these mutant genes may drive the older versions
out of existence.
Thanks to the discovery of DNA, it is now possible for scientists to identify not just the genes, but the individual bases. Before
the discovery of DNA, scientists could only uncover the evolutionary tree of life by comparing the bodies and cells of different
species. Now they can compare their genetic codes, working their way down to the deepest branches of life dating back
billions of years.
View this article online at:
http://evolution.berkeley.edu/evolibrary/article/history_22
Watson image courtesy of James D. Watson; Crick image courtesy of Christof Koch, California Institute of Technology, Pasadena; DNA structure after an illustration at DOEgenomes.org
Understanding Evolution © 2010 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California
1 di 1
21/04/2010 11.37
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_23&context=...
Radiometric Dating: Clair Patterson
Nineteenth century geologists recognized that rocks formed slowly as mountains eroded and sediments settled on the ocean floor. But they could not say just how
long such processes had taken, and thus how old their fossils were. Darwin had argued that the Earth was immensely old — which gave his gradual process of
evolution plenty of time to unfold. The great physicist Lord Kelvin had countered that the planet was actually relatively young — perhaps 20 million years old. He
came up with that figure by estimating how long it had taken for the planet to cool down to its current temperature from its molten infancy. But Kelvin didn't, and
couldn't, know that radioactive atoms such as uranium were breaking down and keeping the planet warmer than it would be otherwise.
An older Earth
At the dawn of the twentieth century, physicists made a revolutionary discovery: elements are not eternal. Atoms can fuse together to create new elements; they can
also spontaneously break down, firing off subatomic particles and switching from one element to another in the process (see figure, right). While some physicists
used these discoveries for applications ranging from nuclear weapons to nuclear medicine, others applied them to understanding the natural world. The sun was once
thought to burn like a coal fire, but physicists showed that it actually generates energy by slamming atoms together and creating new elements. The primordial cloud
of dust that came to form the Earth contained unstable atoms, known as radioactive isotopes. Since its birth, these isotopes have been breaking down and releasing
energy that adds heat to the planet's interior.
Radioactive
elements
decay,
releasing
particles
and
energy.
Radioactivity also gave the history of life an absolute calendar. By measuring the atoms produced by these breakdowns inside rocks,
physicists were able to estimate their ages (right). And by comparing the ratios of those atoms to atoms from meteorites, they could estimate
how long ago it was that the Earth formed along with the rest of the solar system. In 1956 the American geologist Clair Patterson (left)
announced that the Earth was 4.5 billion years old. Darwin had finally gotten the luxury of time he had craved.
Ancient life
The dates that radioactive clocks have put on evolutionary history are astonishing. Life is well over 3.5 billion years old, and until about 600
million years ago, the planet was dominated by microbes. Radioactive clocks have shown that evolution can change its pace — the Cambrian
Explosion of about 535 million years ago saw the relatively rapid emergence of many major lineages of animals in just a few million years. Mammals, which for
150 million years had been small, rodent-sized creatures, rapidly evolved to massive proportions in the wake of the Cretaceous-Tertiary extinction 65 million
years ago. Geological timekeeping continues to be a lively science, with new methods emerging all the time. Some of these methods have helped to pin down
the evolution of our hominid ancestors; anatomically modern humans evolved about 100,000 years ago. While that's nearly 20 times older than the Earth was
once thought to be, it's a geological eye blink.
Scientists measure the ages of
rock layers on Earth using
radiometric dating.
View this article online at:
1 di 2
21/04/2010 11.37
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_23&context=...
http://evolution.berkeley.edu/evolibrary/article/history_23
Dated rock layers image courtesy of the USGS, Western Region Geologic Information.
Understanding Evolution © 2010 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California
2 di 2
21/04/2010 11.37
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_24&context=...
Endosymbiosis: Lynn Margulis
The Modern Synthesis established that over time, natural selection acting on mutations could generate new adaptations and new species. But did that
mean that new lineages and adaptations only form by branching off of old ones and inheriting the genes of the old lineage? Some researchers answered
no. Evolutionist Lynn Margulis showed that a major organizational event in the history of life probably involved the merging of two or more lineages
through symbiosis.
Symbiotic microbes = eukaryote cells?
In the late 1960s Margulis (left) studied the structure of cells. Mitochondria, for example, are wriggly bodies that generate the energy
required for metabolism. To Margulis, they looked remarkably like bacteria. She knew that scientists had been struck by the similarity
ever since the discovery of mitochondria at the end of the 1800s. Some even suggested that mitochondria began from bacteria that
lived in a permanent symbiosis within the cells of animals and plants. There were parallel examples in all plant cells. Algae and plant
cells have a second set of bodies that they use to carry out photosynthesis. Known as chloroplasts, they capture incoming sunlight
energy. The energy drives biochemical reactions including the combination of water and carbon dioxide to make organic matter.
Chloroplasts, like mitochondria, bear a striking resemblance to bacteria. Scientists became convinced that chloroplasts (below right),
like mitochondria, evolved from symbiotic bacteria — specifically, that they descended from cyanobacteria (above right), the lightharnessing small organisms that abound in oceans and fresh water.
When one of her professors saw DNA inside chloroplasts, Margulis was not surprised. After all, that's just what you'd expect from a symbiotic partner.
Margulis spent much of the rest of the 1960s honing her argument that symbiosis (see figure, below) was an unrecognized but major force in the
evolution of cells. In 1970 she published her argument in The Origin of Eukaryotic Cells.
Margulis and others hypothesized that
chloroplasts (bottom) evolved from
cyanobacteria (top).
The genetic evidence
In the 1970s scientists developed new tools and methods for comparing genes from different species. Two teams of microbiologists — one headed by Carl
Woese, and the other by W. Ford Doolittle at Dalhousie University in Nova Scotia — studied the genes inside chloroplasts of some species of algae. They
found that the chloroplast genes bore little resemblance to the genes in the algae's nuclei. Chloroplast DNA, it turns out, was cyanobacterial DNA. The DNA in
mitochondria, meanwhile, resembles that within a group of bacteria that includes the type of bacteria that causes typhus (see photos, right). Margulis has
maintained that earlier symbioses helped to build nucleated cells. For example, spiral-shaped bacteria called spirochetes were incorporated into all organisms
1 di 2
21/04/2010 11.38
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_24&context=...
that divide by mitosis. Tails on cells such as sperm eventually resulted. Most researchers remain skeptical about this claim.
It has become clear that symbiotic events have had a profound impact on the organization and complexity of many forms of life. Algae have swallowed up bacterial partners, and have
themselves been included within other single cells. Nucleated cells are more like tightly knit communities than single individuals. Evolution is more flexible than was once believed.
Phylogenetic analyses based on genetic sequences support the endosymbiosis
hypothesis.
View this article online at:
http://evolution.berkeley.edu/evolibrary/article/history_24
Margulis image by Jerry Bauer; Chloroplast image courtesy of New Mexico State University Electron Microscopy Laboratory; Cyanobacterium image courtesy of the University of Tsukuba Institute of Biological Sciences; Mitochondria image courtesy of the CDC, Public
Health Image Library; Typhus-causing bacteria (Rickettsia) image © David H. Walker and Vsevolod Popov, authors. Licensed for use, ASM MicrobeLibrary
Understanding Evolution © 2010 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California
2 di 2
21/04/2010 11.38
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_25&context=...
Evolution and Development for the 21st Century: Stephen Jay Gould
With the fall of Ernst Haeckel's Biogenetic Law in the 1920s, the evolutionary study of embryos receded into the intellectual backwaters for decades. Haeckel's notion that ontogeny
recapitulates phylogeny was deeply flawed, but it was at least straightforward. The few researchers who tried to carry on the study of embryos and evolution proposed a confusing jumble of
different kinds of evolutionary change — for which they invented a jumble of hideously confusing names such as paedomorphosis, proterogenesis, and phyloembryogenesis. Most
embryologists chose instead to focus on understanding how embryos develop — a formidable question in itself — without thinking much about the evolutionary implications of their work.
Meanwhile, evolutionary biologists concentrated much of their efforts on the blossoming field of genetics.
Evo meets devo again
More than anyone else, the Harvard paleontologist Stephen Jay Gould (left) drew attention back to embryos as evolutionary time capsules. In his landmark 1977 book
Ontogeny and Phylogeny, Gould documented the history of scientific research that had led to so much confusion. But he also demonstrated that the wealth of cases could be
organized by some simple principles. Imagine that the timing of development is controlled by two knobs like you'd find on a radio. One controls the rate at which an
organism grows. The other controls the rate at which it changes shape over time. Random mutation may end up changing the settings of each knob, thereby speeding up or
slowing down the rate at which a species' embryos develop. These kinds of adjustments can alter the entire body of an organism, or individual organs.
If evolution had slowed the rate of shape change of a salamander, but kept everything
else the same, we would have ended up with the axolotl.
Genetic triggers for developmental change
1 di 2
21/04/2010 11.38
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_25&context=...
These changes in timing, known collectively as heterochrony, have proved to be numerous and significant. But Gould knew very well that the ultimate
explanation for heterochrony would be found not in metaphorical radio knobs but in the genes whose effects those knobs represented. Around the time that
Ontogeny and Phylogeny was published, biologists began to isolate genes involved in development for the first time. Since then they've gotten a much
better look at how these genes send signals that trigger other genes, and how they induce embryonic cells to proliferate, die off, crawl to new locations or
stick together.
At the dawn of this new scientific age, Gould predicted that heterochrony and similar evolutionary changes would not be directed by the genes that actually
build various body parts. Instead, the genes that regulate other genes would hold the key to the evolution of embryos. His prediction has now been borne
out. In 2000, for example, Junhyong Kim and his fellow Yale biologists compared the timing at which a crucial developmental gene (see photos, right)
became active in the fruit fly, Drosophila melanogaster, and two closely related species, D. simulans and D. pseudoobscura. They found that the gene
started to make its proteins 24 minutes later in D. pseudoobscura than D. melanogaster. Meanwhile, D. simulans gets a head start: its gene becomes active
14 minutes earlier. And that change led to differences in their anatomy — even though the developmental gene itself is identical in all three species.
As scientists have begun to isolate these regulatory genes, they've been shocked at how powerful they are and how long they've been in power over the
course of evolution.
Developing Drosophila embryo
expresses the hairy gene (dark bands)
over the course of its first four hours of
life. Four stages of this expression are
shown in the sequence above. Different
species turn on the gene at slightly
different times. In frame D, an arrow
marks the furrow that will eventually
separate the head from the rest of the
body.
View this article online at:
http://evolution.berkeley.edu/evolibrary/article/history_25
Gould image courtesy of Jon Chase/Harvard News Office, © 1997 President and Fellows of Harvard College; Salamander image (Pseudotriton ruber ruber) © 2002 John White; Axolotl (Ambystoma mexicanum) image © 2003 Jessica Miller; Drosophila embryo
images courtesy of Junhyong Kim, University of Pennsylvania
Understanding Evolution © 2010 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California
2 di 2
21/04/2010 11.38
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_26&context=...
Genetic Similarities: Wilson, Sarich, Sibley, and Ahlquist
To investigate how birds are related to one another, a biologist of the 1950s would have carefully studied their anatomical similarities and differences. But today, a scientist working on the
same problem could also use the very instructions from which that anatomy was built: its genetic code. DNA sequences form the hereditary links between generations, so it is no surprise that
scientists investigating evolutionary relationships have sought to get closer and closer to the DNA that underlies those relationships. However, reading the genomes of entire organisms did not
fall immediately from the discovery of DNA in the 1950s. In small steps, scientists came closer to their target.
Scientists first began to zoom in on gene sequences by studying the products of DNA: proteins. After all, if two species are closely related, they should have similar gene sequences, which
should then make similar proteins. So before the 1970s, proteins were used as stand-ins for genes in studying evolution.
Testing similarity using antibodies
One way that researchers assessed protein similarities was by harnessing the immune system's ability to recognize foreign proteins. For example, the
immune system of a rabbit will recognize a human protein as foreign and will mount an attack against it by making antibodies specific to that protein. If
those same rabbit antibodies are exposed to a similar protein — from a chimpanzee, perhaps — they will attack it as well. The more similar the proteins
from the two species (human and chimpanzee) are, the stronger this second attack will be. Although variations of this technique were being employed as
early as 1904, more sensitive protocols were developed in the 1960s. These more sensitive techniques revealed the remarkable similarity between the
proteins of humans and those of other great apes. Expanding upon the work of others and making the assumption that fewer protein differences
corresponded to shorter times of separation, Vincent Sarich (above left) and Allan Wilson (above right) estimated that humans, chimpanzees, and gorillas
shared a common ancestor only 5 million years ago — a much shorter length of time than was commonly accepted at the time.
Times of divergence and
phylogeny of hominoids, as
estimated from immunological
data.
1 di 3
21/04/2010 11.38
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_26&context=...
Testing similarity using DNA
Scientists studying the chemistry of DNA moved even closer to actual sequences. Charles Sibley (left) and Jon Ahlquist pioneered the
use of DNA kinetics to investigate evolutionary relationships using a technique called DNA-DNA hybridization (see figure, right). Each
DNA molecule is made of two strands of nucleotides. If the strands are heated, they will separate—and as they cool, the attraction of the
nucleotides will make them bond back together again. To compare different species, scientists cut the DNA of the species into small
segments, separate the strands, and mix the DNA together. When the two species' DNA bonds together, the match between the two
strands will not be perfect since there are genetic differences between the species — and the more imperfect the match, the weaker the
bond between the two strands. These weak bonds can be broken with just a little heat, while closer matches require more heat to
separate the strands again.
DNA hybridization can measure how similar the DNA of different species is — more similar DNA hybrids "melt" at higher temperatures. When this
technique was applied to primate relationships, it suggested that humans and chimpanzees carried DNA more similar to one another's than to orangutans'
or gorillas’ DNA.
Hypothesized evolutionary relationships between
humans and their close relatives based on
DNA-DNA hybridization data.
Sequencing DNA
2 di 3
21/04/2010 11.38
evolution.berkeley.edu
http://evolution.berkeley.edu/evolibrary/print/printable_template.php?article_id=history_26&context=...
The first DNA sequencing methods were invented in the late 1970s, but pure DNA, ready for sequencing, was difficult to produce — thus, making DNA sequencing
labor- and time-intensive compared to other tools for making indirect inferences about genetic sequences. However, in the late 1980s, scientists developed a
technique for producing many, many copies of a very small amount of DNA, and this invention sparked an explosion in the study of DNA sequences. Researchers
began to rely upon sequences as a crucial source of evidence for evolutionary relationships.
Sequencing genes seems to become easier every day. Ten years ago, it might have taken an hour to sequence 10 base pairs. Today a typical lab can sequence
100 base pairs in an hour and facilities with the latest technology sequence hundreds of base pairs each minute. We are now awash in genetic code — we have a
basic map of the human genome and the genomes of many other organisms. However, DNA sequences alone do not answer all the questions that biologists ask,
and knowing a gene's sequence is still many steps away from understanding how it actually works and what it does. DNA sequences are only one line of evidence
illuminating evolutionary relationships. For example, human and chimpanzee DNA is 98% identical, and genetic sequencing can tell us exactly where in the
genome those few DNA differences are — but anatomical, behavioral, and developmental studies are also crucial in deeply understanding our differences,
similarities, and shared evolutionary history.
Machines that automatically
sequence DNA have made
those sequences readily
available for evolutionary
research.
View this article online at:
http://evolution.berkeley.edu/evolibrary/article/history_26
Sarich image courtesy of UC Department of Anthropology, Berkeley; Wilson image courtesy of the Allan Wilson Centre for Molecular Ecology and Evolution; Sibley image courtesy of Thayer Birding Software; Sequencer image courtesy of Sequenom
Understanding Evolution © 2010 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California
3 di 3
21/04/2010 11.38